Why Hot Metal Glows
📖 Why Hot Metal Glows
Figure 1: A heated metal bar — the colour shifts from dark (cool) through red, orange, yellow to white-hot. The colour tells you the temperature — that's black body radiation.
Just like a mood ring changes colour based on your body heat, hot objects change their glow colour based on temperature. Cool objects glow invisibly in infrared (you can't see it but thermal cameras can). Heat something up and it starts glowing dull red, then orange, yellow, and finally brilliant white-blue. The colour tells you the temperature — that's why blacksmiths know exactly when metal is ready to forge!
Definition: A black body is a theoretical perfect absorber and perfect emitter of radiation.
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🎯 Exam Focus
Frequently Examined
This topic is examined in Edexcel 1PH0/1 (Paper 1). Edexcel questions often present a data set — such as radiation spectra for two stars or the Earth — and ask you to compare temperatures using peak wavelength, or to explain the greenhouse effect mechanism in a given context. Expect "Suggest" and "Explain" command words worth 3–4 marks. Common question types:
- Define a black body (2 marks) — must name both properties: perfect absorber AND perfect emitter
- Describe how radiation emitted changes with temperature (3 marks) — total increases, peak wavelength decreases, peak frequency increases
- Explain the greenhouse effect (4 marks) — absorption, re-emission in all directions, less escaping to space, temperature rise to new equilibrium
- Compare two objects using peak wavelength data (2 marks) — shorter peak wavelength = higher temperature
- Radiation balance / Earth's temperature (3 marks) — absorbed solar = emitted infrared at stable temperature
The most common mistake: saying greenhouse gases "block" incoming sunlight. They absorb the infrared radiation emitted by the Earth, not the incoming solar radiation.
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Quiz · Question 1 of 16
What is a perfect black body?
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This topic in real past papers
Every real exam question we've found on black body radiation, with a full worked answer.